With a reduction in the size of semiconductor devices (i.e., semiconductor integrated circuits), it has become difficult to form circuit structures in ideal shapes. Therefore, exposure simulation and circuit designing through optical proximity correction (OPC) have become of increasing importance. In the following description, the “optical proximity correction” is abbreviated to “OPC.” The “exposure simulation” means an optical simulation of an exposure step that is conducted in forming a semiconductor circuit on a substrate. The “designing through OPC” means circuit designing that is conducted by taking into consideration the manufacturing conditions, in particular, the exposure conditions of semiconductors.
For both the “exposure simulation” and circuit “designing through OPC,” it is necessary to inspect the optical properties of an exposure system in advance. During the inspection conducted in advance, representative values, such as the dimensions and shape, of the actually produced semiconductor structure are measured using a length-measuring scanning electron microscope. Usually, measurement procedures of a length-measuring scanning electron microscope are registered in a file called a recipe in advance, so that the length-measuring scanning electron microscope executes automatic measurement in accordance with the recipe.
By the way, automatic measurement of a FEM (Focus Exposure Matrix) wafer, which is conducted using a length-measuring scanning electron microscope to inspect the optical properties of an exposure system, has the following problems.
A first problem is that the size of a semiconductor structure to be measured differs depending on the exposure conditions. When identical semiconductor structures are produced under a plurality of different exposure conditions, the resulting semiconductor structures will have different widths and heights. In the automatic measurement, if the size of a structure to be measured is different from that registered in a recipe, it would be difficult to execute position collation (i.e., pattern matching) through image processing.
The second problem is that if a structure to be measured is not formed, erroneous measurement may be executed. In order to inspect the optical properties of an exposure system, a semiconductor structure is produced in the stage where the exposure conditions are not determined yet. Therefore, there may be cases where semiconductor structures cannot be formed depending on the exposure conditions used. Usually, a length-measuring scanning electron microscope captures an image of a structure to be measured, and outputs the dimensions of the structure calculated through software processing of the captured image. Because of such a mechanism, the length-measuring scanning electron microscope calculates values even when there is no structure formed in the image-capturing region. That is, the length-measuring scanning electron microscope calculates erroneous dimensions as the measured values.
Solutions to the first problem are described in Patent Literatures 1 and 2, for example. Patent Literatures 1 and 2 each disclose a method of allowing pattern matching to be performed on measurement targets with different sizes. A solution to the second problem is described in Patent Literature 3, for example. According to such patent literature, when a shape deformation is detected, pattern matching is determined to fail, and thus, execution of measurement is prohibited.